Byo-yomi is a form of overtime used in Go and in other games popular in Japan, such as shogi. Literally, it means "second reading." In this time system, a player is given a certain amount of main time, usually ten minutes to an hour, and a "byo-yomi time," typically twenty seconds to a minute, and a certain number of byo-yomi "periods." The idea is that the bulk of the game will take place in the main time, but neither player will run out of time if they have efficient time management skills. When his main time runs out, a player passes into his byo-yomi periods, where he perpetually has the same amount of time per move as long as he makes moves within that allotted time. If a player doesn't make a move in the allotted time, he loses a byo-yomi period, but not the game. When byo-yomi periods reach zero, the player loses on time.

In an actual game, this usually plays out such that the first 150 moves or so are played in the player's main time--perhaps an average of 2.5 moves per minute over an hour' s time. After this time has been used, the player's byo-yomi kicks in, and he must play a move in thirty seconds, or lose a period. After all the periods have been used, the player loses on time. This introduces the problem of time pressure into the part of the game played under byo-yomi, since move importance is given no weight. That is, even if a player played faster, on average, in his main time than the byo-yomi requires, chances are he spent a minute or more on a few moves while answering other moves in a few seconds, on basic instinct. Because of this difference, even strong players often make grave mistakes when playing under time pressure.

In the west, byo-yomi has a much looser meaning among go players. The strict definition given above refers to what western players often call "Japanese byo-yomi," which is rather redundant. Other systems, such as "absolute time" (games played without an overtime allotment) and "Canadian byo-yomi" (a rather chess-like overtime system wherein a certain number of moves are required in a certain time frame), are commonly used on the internet go servers.